Silicon carbide is a very hard, strong, inert ceramic material, well known in the fields of abrasives and refractories, which now is also being employed for monolithic or composite structural parts in an increasing number of advanced aerospace projects. The current invention is outside these fields and is directed instead to means for producing thin silicon carbide coatings and films. The coatings and films can be applied to various articles for protective and other purposes. In general, silicon carbide coatings and films can be produced by chemical vapor deposition, sputtering, sol gel coating, plasma spray, or reactant spraying. Of the available methods, non-reactive sputtering is the focus of this invention.
In general, the sputtering process is carried out in a chamber containing a selected gas, such as argon, under reduced pressure. The process involves the creation and acceleration of ions toward a target material. The ions may be created by applying a high voltage across the gas in the chamber, producing a plasma of projectile ions, such as Ar.sup.+, and electrons, but the projectile ions can be generated by other means as well, such as a separate ion gun. The ions, accelerated in the electric field, strike the target, and their momentum is transferred, dislodging atoms or molecules from the target surface. The mechanically dislodged, predominantly electrically neutral molecules or atoms then deposit themselves on a desired substrate, generally placed within a line of sight from the target.
The production of silicon carbide films and coatings by use of sputtering processes has been described in the prior art. One aspect of the current invention is a silicon carbide sputtering target which can be employed in sputtering processes. Gates, et al., U.S. Pat. No. 4,209,375 discloses a sputtering target assembly which may include a SiC sputtering target.
Schmatz, et al., U.S. Pat. No. 4,936,959 discloses a process for producing an amorphous coating of SiC on a metal tool by directing a beam of argon ions from a narrow beam ion source onto a silicon carbide target maintained at a bias potential of about zero. The SiC deposition rate was very low, requiring at least 300 min. to deposit a film 1-4 micrometers thick.
Hoffman, U.S. Pat. No. 4,737,252 discloses the deposition of thin protective films of various inorganic non-metals, including SiC, onto the surfaces of metal articles, such as jewelry, using an RF sputtering technique. The RF-sputtered films of silicon carbide had a yellowish-brown tint, rendering them of marginal interest in that and other applications where the optical properties of the films are important.
The RF sputtering technique is especially useful for producing films of materials which are electrical insulators or semiconductors. Pure silicon carbide is a semiconductor; its room temperature electrical resistivity is about 10 ohm-cm; see W. Kingery, et al., "Introduction to Ceramics," 2nd Ed., Wiley-Interscience, New York, N.Y., 1976, p. 851. One disadvantage of RF sputtering is the relatively low rate at which films of the sputtered material can be built up, and a second disadvantage is excessive substrate heating.
Other sputtering techniques are available if the target is more electrically conductive. For example, DC cathode sputtering can be employed to deposit films or coatings of metals. One disadvantage of DC cathode sputtering is that, as the sputtering rate is increased by increasing the electric power, a good deal of heat is generated. Excessive heat can destroy the substrate, e.g., a plastic substrate.
The electrical conductivity of silicon carbide can be increased by doping the silicon carbide with excess carbon, in effect producing a "non-stoichiometric silicon carbide", also termed "SiC.sub.x " hereinafter, in which x is the molar ratio of carbon to silicon and is greater than 1. The non-stoichiometric silicon carbide is generally more electrically conductive than undoped SiC and often can be machined by electrical discharge methods.
Such non-stoichiometric silicon carbide is disclosed in Boecker, et al., U.S. Pat. No. 4,525,461, which is incorporated herein by reference. The Carborundum Company, assignee of this application, markets such silicon carbide products under the trade name HEXOLOY.RTM. SG silicon carbide. The '461 patent discloses sintered silicon carbide ceramic bodies composed of silicon carbide in combination with about 1-48 percent by weight ("wt %" hereinafter) graphite, with about 1.5-49 wt % total uncombined carbon, which includes any amorphous carbon which may also be present, the lowest total uncombined carbon content thus being about 1.5 wt % of the ceramic body. The sintered silicon carbide ceramic bodies disclosed in the '461 patent also contain a sintering aid residue, about 0.15-5.0 wt % based on the amount of silicon carbide. The chemical compositions of some of the sputtering targets of this invention and some raw batches leading to the sputtering target of this invention are within the disclosure of the '461 patent.
It is disclosed in Funkenbusch, U.S. Pat. No. 4,917,970 that a commercially available HEXOLOY.RTM. SG non-stoichiometric silicon carbide product, i.e., SiC.sub.x can be very successfully employed as a sputtering target to produce protective coatings on magneto-optic recording media. The "x" was estimated to be 1.47 in the commercial SiC.sub.x product, based on the Auger electron emission spectrum of a sputtered film. An "x" value of 1.47 corresponds to a SiC.sub.x film containing about 12.3 wt % excess, uncombined, or free carbon.
A sputtering variation called DC magnetron sputtering, in which a magnetic field is oriented perpendicular to the DC electric field, decreased the substrate heating to such an extent that even plastic substrates could be coated with films of the non-stoichiometric HEXOLOY.RTM. SG silicon carbide.
In magneto-optic applications, the optical transmittance of the silicon carbide coating should be as high as possible at the wavelengths of interest. This permits the use of thickened silicon carbide films, providing optimum protection to the substrate. Windischmann, et al., U.S. Pat. No. 5,190,631 describes a method for improving the optical transmittance of sputtered silicon carbide films by adding some hydrogen to the gas in the sputtering chamber.
Funkenbusch, U.S. Pat. No. 5,158,834, a continuation of the aforecited '970 patent, recognizes a special benefit in the use of silicon carbide protective coatings in magneto-optic devices. The refractive index of silicon carbide is high enough that the amount of light reflected at a silicon carbide/air interface makes it unnecessary to include a separate reflective layer in the construction.